1. Field of the Invention
The present invention relates to electron guns with grids, especially those used to equip high-power electron tubes. These tubes are chiefly of the linear beam type, such as klystrons or travelling-wave tubes. They could also be triode or tetrode type tubes.
The guns of linear-beam tubes comprise a cathode made of an emitting material and an anode taken to a potential that is more positive than that of the cathode.
2. Description of the Prior Art
When the cathode is heated, it gives an electron beam that is directed towards the anode. To obtain a variation of the intensity of the electron beam in time, a control grid positioned between the cathode and the anode is used. It is taken to an intermediate potential between that of the cathode and that of the anode.
The control grid is bombarded by the electrons. There is an interception of the electron beam that causes major difficulties in high-power tubes. Furthermore, the control grid gets greatly heated, thus restricting the time for which the gun can be used. To eliminate the bombardment of the control grid, it is proposed to place a shadow grid in the vicinity of or in contact with the cathode. This shadow grid has the same pattern as the control grid and is often taken to the potential of the cathode. But it has been seen, in the course of use, that this shadow grid, in getting heated like the cathode, itself becomes emissive as a result of a migration of the emissive matter from the cathode. The electrons emitted by the shadow grid have aberrant paths and the electron beam converges poorly. These electrons strike the control grid and/or other constituent elements of the tube in an uncontrolled and undesirable way. In the configurations where the shadow grid is in the vicinity of the cathode, there is less risk of pollution of the shadow grid by the emissive material. However, the emission of electrons is equally well disturbed for the entire zone located between the emissive surface and the shadow grid is at the same potential.
In other configurations again, a shadow grid has been associated with circular apertures and a cathode having an emissive surface which, facing each aperture of the shadow grid, is shaped like a spherical dome. The electron beam sent is less disturbed, but the making of a gun such as this is more complicated and costlier.
The present invention proposes to overcome these drawbacks. It proposes the elimination of the shadow grid and the hollowing out in the cathode of grooves that are deep enough so that their bottom does not emit electrons. The absence of the shadow grid eliminates pollution. The grooves with a non-emissive bottom form a virtual shadow grid. The grooves are positioned so that they directly face the solid parts of the control grid. The electrons emitted by the cathode do not bombard the control grid.